Fractional freeze separation apparatus and process



Nov. 19, 1968 J. K. SKREBOWSKl ET AL 3,411,309

FRACTIONAL FREEZE SEPARATION APPARATUS AND PROCESS Filed Feb. 15, 1967 3M4 r W M United States Patent 3,411,309 FRACTIONAL FREEZE SEPARATION APPARATUS AND PROCESS Jerzy Konrad Skrebowski and John Williamson, Nortonon-Tees, England, assignors to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain Filed Feb. 13, 1967, Ser. No. 615,510 17 Claims. (CI. 62-58) ABSTRACT OF THE DISCLOSURE Liquids (for example mixtures of xylenes) are frozen to produce slurries of crystals and mother liquor by contacting the liquid with a cold surface. Crystals which adhere to the surface are removed by applying sonic vibrations to the liquid.

This invention is concerned with a refrigeration process.

In processes for freezing liquids, for example those comprising paraxylene and at least one other xylene and/0r ethyl benzene, to produce slurries comprising a mother liquor and crystals, by contacting them with a cold surface, there is a tendency to build up solid deposits on the chilled surface, which results in inefficient heat transfer and failure to recover the crystal products rapidly from the system. It has previously been proposed to provide mechanical scrapers to clean the chilled surface, but such systems are often expensive to maintain and produce slurries containing mainly small crystals especially in the case of paraxylene crystals. We have now devised means whereby this problem may be significantly reduced.

According to our parent British complete specification in patent application Nos. 35,367/65, 9,187/66 and 29,226/ 66 this difficulty may be overcome, when p-xylene slurries are produced, by subjecting the chilled surface to sonic vibrations. This is a most effective procedure, but suffers from the disadvantage that sonic vibrations are easily lost from the surface to parts of the apparatus by which it is supported, and it is normally necessary to isolate the surface accoustically.

The present invention is based on the discovery that crystals may be very effectively dislodged from the chilled surface by subjecting the liquid undergoing chilling by contact with the surface to sonic vibrations, and that such vibrations largely bounce off i.e are reflected from, the surface, which therefore need not vibrate to a sufiicient extent to render accoustical isolation necessary.

The invention therefore provides a process of freezing a liquid to produce a slurry comprising crystals and mother liquor which comprises contacting the liquid with a cold surface at a sufficiently low temperature to cause the formation of crystals on the cold surface and detaching crystals from the cold surface by applying sonic vibrations to the liquid at a point removed from the cold surface. (The term sonic vibrations in this application includes vibrations above the audible range.) The frequency of the vibrations is at least 40 cycles per second and is preferably in the range of 0.5 to 100 kilocycles per second. Suitably, vibrations of a low frequency of from 5, and preferably 10, to 30 kilocycles per second are used as these tend to produce a high turbulence in the layer of liquid close to the cooling surface and tend also to be easily produced at a high power.

The invention also provides apparatus for carrying out the said process.

It is preferred to apply the vibrations at a small angle of at most 20 to the cold surface, so as to increase the 3,411,309 Patented Nov. 19, 1968 proportion of the vibrations reflected. It is especially preferred that the vibrations should be parallel to the surface. We have found the use of such small angles to be very effective in preventing incrustation of the cold surface.

The invention is especially suitable for producing slurries from liquids which are mixtures of organic compounds. It is preferred, when the liquid comprises paraxylene that it should comprise at least 10% and preferably at least 15%, to by weight of paraxylene the remainder of the liquid comprising one or more other xylenes and/ or ethyl benzene.

The cold surface is a thermally conductive barrier preferably separating the liquid to be chilled from a refrigerant.

We have found that the process of this invention permits at the same rate of heat exchange and under the same process conditions, the production of larger crystals than is possible if only mechanical scraping is used to free the surface of crystal deposits. We have also found that a higher rate of heat exchange can be achieved with the same temperature difference between the chilled surface and the liquid.

Suitable cold surfaces may be provided, for example, by using one or more tubes of which the walls form thermally conductive barriers, the said tubes either conducting the liquid to be chilled through a bath of refrigerant or conducting a refrigerant through a vessel containing the liquid to be chilled. Such tubes should preferably have an internal diameter of not less than one quarter and more preferably not less than half an inch.

Plate and corrugation heat exchangers may also be used.

A particularly satisfactory form of this invention is obtained by providing a second surface facing the cold surface the liquid being between the said surfaces, and transmitting sonic vibrations substantially parallel to the cold surface. The second surface serves to reflect diverging sound waves onto the cold surface. The second surface may itself be a cold surface in order to increase the total freezing effect, but a highly efficient process may be obtained without this.

One apparatus according to the invention comprises two, preferably substantially concentric, straight tubes one of which contains the liquid to be frozen and the other of which contains refrigerant, and means to apply through the liquid to be frozen, sonic vibration longitudinally to the cold surface contacting the liquid to be frozen, the inner tube being of thermally conductive material. The outer tube very suitably contains the liquid to be frozen. The inner tube may be closed at one end, thus forming a finger, or may be open at both ends. If a finger is used and the vibration is transmitted from beyond the closed end it is preferred that the closed end should be pointed, as we have found that flat or rounded ends tend to ice up, whereas pointed ends are not only more satisfactory in this respect, but the vibrations reflected from them are re-reflected from the outer cylindrical tube and exercise a de-icing effect further up the finger. It is, however not essential to have a pointed end, and for ease of fabrication, rounded or flat ends may be preferred.

An advantage of the use of a finger is that the sonic vibrations need be generated over a small cross-sectional area only, being directed towards the end of the finger, whereas with tubes of which both ends are open it is desirable to provide an annular sonic vibrator or several individual sonic vibrators round the tube so as to avoid shadowing of parts of the cold surface.

A finger may if desired be secured rigidly at its open end, suitably by means of a flange, to other parts of the apparatus, for example the vessel containing liquid undergoing treatment. The finger may be fed with refrigerant, if a downwardly extending finger is used, by uniting the open end of the finger to a refrigerant reservoir.

The refrigerant may be pumped to the closed end of the finger through an internal tube, but this is normally unnecessary with downwardly extending fingers as, if the tube has a sufficient diameter (for many purposes an internal diameter of 4 inches is ample if the finger is, for example, five feet long), liquid refrigerant flows into the tube under the influence of gravity at a quite satisfactory rate despite the effect of boiling.

One suitable form of apparatus according to the invention comprises a bath for the liquid to be frozen, a plurality of cylindrical fingers dipping into the bath, the fingers being either close together and parallel so that sonic vibrations are reflected from one to another or being surrounded by cylindrical sheaths substantially concentric with and spaced away from the fingers, and at the base of the bath sonic vibrators directed towards the base of the fingers. Alternatively, for ease of construction, fiat separators may be provided in the liquid between the fingers.

Refrigerants for cooling the chilled surface may be cooled in any refrigeration device, which may be of conventional design. The refrigerant may be for example, ammonia, ethylene, ethane or carbon dioxide. Suitable refrigerants however when only moderately decreased temperatures are required are brine, petrol, methanol and acetone, or preferably a pentane, for example n-pentane.

A number of the refrigerants for chilling the surface (for example ammonia, ethylene, ethane and carbon dioxide) normally evaporate in the chilling process, and thus produce a boiling effect, though this can be suppressed by using high pressures. Refrigerants of this type are particularly appropriate for the production of low temperatures, which are used in the treatment of mixtures of xylenes (which may contain ethyl benzene) containing only small concentrations (for example, of 10%- 30%, and more usually l5%-25% by weight) of pxylene.

The formation of p-xylene crystals occurs satisfactorily on flat, concave or convex surfaces.

Suitable chilled surfaces are of any material resistant to the temperatures employed in the process, which has a high thermal conductivity, for example, aluminum and its alloys, many copper alloys, especially the copperberyllium alloys and brass, but preferably steels, for example stainless steel. Preferably, the surface is smooth, and it is more preferably polished.

The source of the sonic vibrations may be for example a piezo electric device, a liquid siren or a generator for electric current at a frequency of from 0.5 to 100 kilocycles per second, together with a coil connected across the generator and surrounding a core of magneto-strictive material, one end of the core being contacted either directly or indirectly, for example through a velocity transformer to amplify the vibrations, with the liquid to be frozen.

Preferably the sonic vibrations have a power of, for example, 5 to 200 watts per square foot of the cold surface. At least watts per square foot and at most 100 watts per square foot are usually used.

The cold surface may be at a temperature in the range of 0.5 to 30 or even 50 Centigrade degrees and preferably 5 to centigrade degrees below the temperature of crystallisation of the liquid. A preferable temperature difference is 10 to 15 C. In general, the greater the temperature difference, the greater the power of sonic vibration required. The greater the concentration of p-xylene in mixtures comprising at least one other xylene and/or ethyl benzene, the smaller the temperature difference which is practical.

It is preferred that the process of the invention be carried out whilst feeding the liquid and withdrawing the slurry continually.

The liquid to be frozen may if desired be stirred, but it is more preferably pumped past the cold surface.

It is preferred that the linear flow rate should be at least six inches per second, and that flow should be turbulent. If a liquid siren is used as a sonic vibrator it has the advantage of creating its own turbulence.

It is advantageous to pass the slurry produced into a settling tank so as to permit the withdrawal of a thickened slurry from the base and to allow any possible increase in crystal size thus facilitating subsequent separation of the crystals. At least part of the thinner part of the slurry in the settling tank may be recycled to the liquid contacting the cold surface so as to seed the liquid and thus further to increase the crystal size.

Intermittent sonic vibrations may be applied according to the invention but it is preferred to apply them continuously during the process in order to prevent the build up of incrustations.

By the use of this process slurries containing or more by weight of solids may be produced, though for handling purposes slurries containing at most 40%, for example 5 to 25% by weight of solids are normally preferred.

In all forms of the invention it is preferred when the weight of vibrating parts of the apparatus is supported rigidly, to provide the support at nodal points, if such points exist.

The crystallised material may be recovered by any suitable method, for example, by filtering or centrifuging the slurry to separate the crystals, or the slurry may be treated in known manner to recover the crystallised material as a liquid by for example feeding the slurry to a pulsed column apparatus of conventional type such as that as described by Marwil and Kolmer, Chemical Engineering Progress 59 (February 1963) page 60*.

Example 1 The apparatus will now be described with reference to the drawing.

A three inch external diameter, 13 /2 inch long stainless steel round bottomed tube 1 was mounted in a 4" internal diameter glass column 2 holding a mixture of xylenes and ethyl benzene containing by weight of paraxylene. 9 /2 inches of the tube were immersed in the mixture. A 2 diameter velocity head 3 was fixed at its nodal (mid) point in the base of the glass column so that the free end faced the bottom of the tube and was 4% inches from it. A thermometer 7 was provided dipping into the liquid mixture, and tubes 5 and 6 were provided for circulating (if required) liquid through the glass column. The velocity head 3 was vibrated at 13 kilocycles/ sec. by three magnestostrictive transducers 4 driven by a nominal lOO-watt electrical generator. No attempt to optimise the efficiency of conversion of electrical to mechanical energy was made. Cold methanol was circulated through the tube by means of pipes 8 and 9, a constant depth of methanol of 9 /2" being maintained in the tube.

With the xylenes unagitated, and at a heat flux of 350 kilocalories/hn/sq. ft. (corresponding to the maximum refrigeration capacity for the cold methanol), using sonic vibrations, no ice adhered to the surface of the tube. A slurry containing 18% by weight of paraxylene crystals was produced.

Without sonic vibrations the tube iced up as soon as the freezing point of the liquid was reached. The above procedure was repeated, circulating the mixture of xylenes and ethyl benzene through the glass column through pipes 5 and 6. Slurry formation was possible without forming surface ice up to a heat flux of 450* kilocalories/ sq. ft./ hr. using sonic vibrations. Without vibrations, the maximum heat flux which did not cause icing up was kilocalories/hr./sq. ft.

Example 2 The apparatus used in this experiment was similar to that of Example 1, except that nozzles were provided through which the inner walls of the tube were sprayed with acetone and a vacuum was applied to the inside of the tube to evaporate the acetone. The mixture of xylenes and ethyl benzene was again circulated. No icing of the tube surface was observed at a heat flux of 470 kilocalories/ sq. ft./ hr. using sonic vibrations as before.

Without sonic vibrations the maximum allowable heat flux was 80 kilocalories/sq. ft./hr.

Example 3 The tube of Example 2 was filled to a depth of 9 /2" with a commercially available refrigerant boiling at a temperature of 30 C. at atmospheric pressure and sold under the trade mark Arcton 12. The refrigerant was then allowed to boil. The maximum heat flux without icing up using sonic vibrations was 480 kilocalories/sq. ft./hr. and the maximum heat flux without the vibration was 100 kilocalories/sq. ft./hr., the mixture of xylenes and ethyl benzene being circulated in both cases.

On repeating this procedure using a 6" nominal bore glass column, the end of the tube being 4 /2 inches from the end of the velocity head, the maximum heat flux with the sonic vibrations was 470 kilocalories/sq. ft./hr. and without the vibration, 8O kilocalories/ sq. ft./ hr.

Using a 6" nominal bore glass column with the end of the tube 17" from the velocity head, the maximum allowable heat flux with the sonic vibration was 400 kilocalories/sq. ft./hr. and without the vibration was 90 kilocalories/ sq. ft./ hr.

We claim:

1. A process of freezing a liquid to produce a slurry comprising crystals and mother liquor, which comprises contacting the liquid with a cold surface at a sufficiently low temperature to cause the formation of crystals on the cold surface wilst detaching crystals from the cold surface by applying sonic vibrations to the liquid at a point removed from the cold surface.

2. A process as claimed in claim 1 in which the vibrations have a frequency in the range of from 0.5 to 100 kilocycles per second.

3. A process as claimed in claim 2 in which the vibrations are applied at an angle of at most 20 to the cold surface.

4. A process as claimed in claim 3 in which the cold surface is provided in the form of at least one tube of which the walls form thermally conductive barriers, the said one tube either conducting the liquid to be chilled through a bath of refrigerant or conducting a refrigerant through a vessel containing the liquid to be chilled.

5. A process as claimed in claim 1 in which the liquid is a mixture of organic compounds.

6. A process as claimed in claim 5 in which the liquid comprises from 10 to of paraxylene by weight, the remainder of the liquid comprising one or more other xylenes and/or ethyl benzene.

7. A process as claimed in claim 1 in which a second surface faces the cold surface, the liquid being between the said surfaces, and in which sonic vibrations are transmitted substantially parallel to the cold surface.

8. A process as claimed in claim 7 in which the second surface is itself a cold surface.

9. A process as claimed in claim 1 in which the liquid is fed and the slurry is withdrawn continuously.

10. A process as claimed in claim 1 in which a refrigerant which evaporates during the chilling process is employed for chilling the surface.

11. A process as claimed in claim 2 in which the sonic vibrations have power of 5 to 200 watts per square foot of the cold surface.

12. A process as claimed in claim 11 in which the temperature of the cold surface is in the range of 0.5 to 30 centigrade degrees below the temperature of crystallisation of the liquid.

13. A process as claimed in claim 12 in which the liquid to be frozen is pumped past the cold surface.

14. A process as claimed in claim 13 in which a liquid siren is used as a sonic vibrator.

15. A process as claimed in claim 13 in which the sonic vibrations are applied continuously during the process.

16. Apparatus suitable for freezing a liquid to produce a slurry of crystals in mother liquor which comprises an inner straight tube of thermally conductive material for containing the refrigerant, an outer tube disposed substantially concentrically around the inner tube to provide an annular space for containing the liquid and means to apply in a longitudinal direction with relation to the tubes sonic vibrations to the liquid in the annular space.

17. Apparatus as claimed in claim 17 in which the inner tube is closed at one end to form a finger.

References Cited UNITED STATES PATENTS 3,266,263 8/1966 Pollock 62123 X 2,595,968 5/1952 McCoy 62-68 2,816,938 12/1957 Hess 62123 X 2,886,603 5/1959 Shelton 260645 2,960,843 11/1960 Zdansk y et al. 62-423 3,224,213 12/1965 Hoyt 62-340 X ROBERT A. OLEARY, Primary Examiner.

W. E. WAYNER, Assistant Examiner. 

